(1) Field of the Invention
The rare earth elements (REEs) are the Lanthanum series elements plus Yttrium, Atomic Number elements 57 through 71 plus 39. This invention provides a method for separating the 10 heaviest lanthanum series elements Sm through Lu plus Y.
This ion exchange project was initiated to find a method other than solvent extraction for separating the heavy rare earth elements. Solvent extraction is the current method used to separate the rare earths, but is best suited to separating the high tonnage light rare earths, cerium subgroup La through Sm plus Eu. The most abundant rare earth, cerium, is precipitated as Ce.sup.+4 when the other rare earths are digested and put into solution in the "+3" oxidation state. Europium is difficult to separate from the other rare earths by solvent extraction, but its high value makes its recovery commercially attractive.
Solvent extraction requires a large capital investment for each REE recovered and requires significant time while the mixer-settlers obtain equilibrium. The heavy REEs, yttrium subgroup Eu through Lu plus Y are more difficult to separate by solvent extraction, and the economy of scale is less favorable.
It was postulated that an improved ion exchange process would be more economical, technically favorable, and environmentally friendly. At the end of World War II, an ion exchange technique called band displacement or displacement chromatography was discovered to separate the rare earths. This process was the first method that produced pure REEs on a commercial scale, but it required many months to produce a product.
(2) Description of Prior Art
The REEs have very similar chemical and physical properties. A number of methods have been used to separate them.
Fractional precipitation takes advantage of differences in the solubility product of the REEs. Carbonate, oxalate, and hydroxide anions precipitate rare earths as ammonium double salts. This method requires many recrystallizations and does not produce a pure product. The heavy rare earths are more difficult to separate than the light rare earths. This method has been abandoned for most rare earth separations.
Solvent extraction is a well known technique of separation. The REEs have a preferential distribution between two immiscible liquid phases. Extractants useful for separating rare earths are: neutral phosphorous agents; monoacidic orthophophosphate and phosphorate esters; and primary, secondary, tertiary, and quatery amine species. An acidic aqueous phase is used in combination with the extractant. After many mixer-settlers contracts, a rare earth can be separated from the other rare earths. High purity requires an extremely high number of mixer settlers. A different plant is required for each REE. The elements in the yttrium subgroup are more difficult to separate than those in the cerium subgroup. Dedicating a solvent extraction plant to each of the heavy REE is difficult to justify on an economic basis because of their low abundance. If Y is excluded from the heavy rare earths, then the combined weight of the five REEs in the cerium subgroup in the earth's crust is five times greater than the combined weight of the nine heavy REEs. Solvent extraction reagents must be replaced when degraded. Environmental laws requires the destruction of organic reagents.
An ion exchange technique called elution chromatography can be used to separate REEs. By this analytical method, mixed rare earths are pumped into an ion exchange column and then an eluent is pumped into the ion exchange column to push the rare earths through the column. Separation is accomplished by each of the REEs traveling through the resin column at different rates. The rare earths are collected as they exit the column. Sulfonic resin and alpha-hydroxyisobutyrate eluent are typically used. In this process, only a small fraction of the resin is occupied by rare earths while the separation is occurring. This process separates a small amount of the rare earth for the amount of capital and skilled labor invested.
Rare earths can also be separated by an ion exchange technique called "displacement chromatography," or referred to as "band displacement". Columns containing ion exchange resin are connected head-to-tail. A length of resin is used to contain the mixed REEs being separated; this is called the "loading column". The term "band of mixed rare earth" refers to the amount of rare earth being separated. The rare earths are saturated on the loading column's resin and are measured as a volume or length if all the columns have the same diameter. The preferred exchange media is sulfonated polystyrene-divinylbenzene copolymer resin (sulfonic resin), which is a strong cation exchange resin. A band of mixed rare earths is saturated on the sulfonic resin in one or more columns (i.e. loading column) from a rare earth salt solution of chloride, sulfate, or nitrate.
Sulfonic resin in a second group of columns called "separation column" is converted to a special cation state by pumping a sulfate, chloride, or nitrate solution of that cation through the resin. The special cation is Cu.sup.+2 or Zn.sup.+2 if ethylenediaminetetraacetic acid (EDTA) is used, or H.sup.+ if hydroxyethylenediaminetriacetic acid (HEDTA) is used. Ammonium EDTA or HEDTA is pumped through the loading column containing the mixed REEs and then through the separation column saturated with the special cation. Ammonium chelating solutions of EDTA and HEDTA are used to strip or elute the REEs. The ammonium associated with the chelating solutions replaces the REE on the loading columns resin. The chelates have a tighter affinity for heavier REEs than lighter REEs. The special cation on the resin acts as a chemical barrier or retaining ion by exchanging places with the REE in solution. This prevents the faster eluting heavy REEs from traveling through the columns at the speed of the EDTA. The lighter rare earth cations in solution exchange places with heavier rare earths saturated on the resin and this promotes the development of purified bands of rare earth elements. Many of the rare earths have distinctive colors that emphasize this chromatic effect.
The number of exchange sites required in the separation column is usually two or three rare earth band lengths of resin for exchange columns with the same diameter. Other mass transfer parameters include eluent velocity, temperature, resin diameter and porosity, which REEs are being separated, etc. A quantity of impure (binary) rare earth occupies the resin between each pure REE in the separation column. What percentage of a given REE is pure depends on mass transfer factors, diameter of column, and amount of that REE being separated. The rare earth solution containing binary REEs is decomposed and the rare earth is recycled to a subsequent band displacement. Eluent containing one REE is precipitated as an oxalate and then burned to produce rare-earth oxide product.
Band displacement requires low elution velocities of about 1 cm/min or lower, to facilitate the mass transfer required for good separation. Higher initial velocities are frequently used to make rough separations, but additional elutions at lower velocities are then required. Odd atomic numbered REEs are scarcer than even atomic numbered REEs. Also, heavier REEs are scarcer than lighter REEs. The absolute length of resin between separated REEs is the same for different diameter columns. For this reason, scarce rare earths are eluted onto small diameter columns to increase rare earth recovery. Smaller diameter columns with the same eluent velocity require longer elution times.
The number of mixed rare-earth band lengths of retaining cations required to obtain a given separation depends on mass transfer parameters. The time required to complete an industrial scale rare-earth separation is about five months. When EDTA is used, it is very difficult to separate and recycle the Cu and EDTA because they are bound very tightly together. Also, Lu bleeds through the retaining ion. If Zn is used as retaining ion, the EDTA and Zn can be separated and recycled, but Lu, Yb, Tm, and Er will bleed through the Zn. HEDTA is easily recycled, but it does not separate the medium weight REEs Sm through Ho.
Resins other than sulfonic were tested as a media to separate REEs. Iminodiacetic resin was tested, but with little success. A gross separation of the heavy/light REEs was reported as the best results achieved. Hydrogen form iminodiacetic resin was used to concentrate Sc from other elements using dilute acids like HCl, and then a chelating eluent was used to clute the Sc.
Of the methods cited, only band displacement shares the same separation scheme as selective elution. This invention describes in detail the differences between band displacement and selective elution. The other methods of separating the rare earth elements are based on different technologies.